For over a hundred years, pathologists have routinely preserved biological samples such as tissue samples by fixing them with formaldehyde. While formaldehyde treatment preserves the cellular features of the tissue, formaldehyde treatment also results in chemical cross-linking that renders many of the biological components of the sample poorly accessible or inaccessible to detection, quantification and characterization. Formaldehyde preserves or fixes tissue or cells by cross-linking primary amine groups in proteins with other nearby nitrogen atoms in protein or DNA through a —CH2— linkage. Thus, for example, while the polymerase chain reaction (PCR) is useful to detect and quantify nucleic acids in biological samples, PCR is generally poorly or not effective in analyzing nucleic acids in formaldehyde cross-linked samples, especially where quantitative results are desired.
Cross-linking of nucleic acids to cellular components by the action of formaldehyde thus presents challenges to the detection of various cellular components, including detection of nucleic acids and proteins. While some have described ways of improving amplification of nucleic acids from formaldehyde cross-linked samples, the improvements generally involve merely degrading protein in the sample or providing detergents that do not generally change the covalent bonds forming the cross-links. The present invention addresses this and other problems.